1. Field of the Invention
The present invention relates to a quality check system for judging as to whether welding quality of a spot welding is good or poor.
2. Description of the Related Art
In spot welding for carrying out resistance welding by pressurizing an object to be welded by using an electrode, welding quality of the spot welding cannot be checked by an appearance of the object. In the prior art, various methods have been proposed as methods for checking welding quality (for example, a method of monitoring a displacement between electrodes, a method of using a voltage or resistance between electrodes, a method of using a ultrasonic wave, a method of measuring the temperature, and a method of using an acoustic emission, etc.)
Among the above methods, the method of monitoring the displacement between the electrodes can be utilized for checking the welding quality of various kinds of material, since an error due to split-flow of welding current does not occur in the method. As a relevant prior art document, JP 2001-300738 A discloses a method of: measuring a degree of thermal expansion of a material to be welded by using an amount of displacement of a value of an encoder attached to an actuator when current is applied to the actuator; measuring a degree of contraction of the material by using an amount of displacement of a value of the encoder before and after current is applied to the actuator; and checking welding quality based on the degree of thermal expansion and the degree of contraction.
Further, JP 2000-005882 A discloses two types of methods for judging welding quality. In the first method, data of a displacement between electrodes when welding quality is good is previously obtained as reference data; and data of displacement between the electrodes during welding after a certain time point is compared to the reference data so as to check the welding quality. In the second method, a saturation point of thermal expansion due to the welding is calculated based on data of displacement between the electrodes during welding; an expansion velocity, an amount of saturated thermal expansion, an expansion time and a contraction time of an object to be welded are calculated based on the saturation point; and these calculated values are respectively compared to an expansion velocity, an amount of saturated thermal expansion, an expansion time and a contraction time, which are previously stored as an ideal model, so as to check welding quality.
In the method of JP 2001-300738 A, if the pressurizing force is purposely changed when applying current, the degree of thermal expansion or contraction cannot be measured. Further, since the degree of thermal expansion or contraction may be changed depending on a material or thickness of the object to be welded, it is necessary to change a parameter for checking welding quality for every welding point.
In the first method of JP 2000-005882 A, it is necessary to adjust two parameters, i.e., a time point and a threshold of deviation. Therefore, it is difficult to determine as to which parameter should be adjusted when a problem in welding quality occurs at a production field. Further, it is necessary to adjust these parameters depending on the material or thickness of the object to be welded.
On the other hand, in the second method of JP 2000-005882 A, in case that the pressurizing force is changed when applying current, the expansion velocity, the amount of saturated thermal expansion, the expansion time and the contraction time cannot be measured. Further, since the degree of thermal expansion or contraction may be changed depending on the material or thickness of the object to be welded, it is necessary to change the threshold with respect to an ideal model for every welding point.
In some cases, a welding machine for carrying out adaptive control may be used, in which current is changed when applying the current. In this case, a waveform of an encoder value when applying the current may be different even between the same welding points of a plurality of the same type of objects to be welded. Therefore, a parameter (or threshold) for checking welding quality cannot be set to a severe value, resulting in that bad or poor welting quality may be overlooked.
As explained above, the prior art includes following three major problems.
(1) Since an evaluation is not carried out based on continuous data, an amount of evaluation cannot be correctly measured in case that the pressurizing force is changed when applying current.
(2) Since it is necessary to adjust the parameter with respect to every welding point, such an adjustment is burdensome.
(3) in the welding machine in which current is adaptively changed when applying the current, the parameter or threshold for checking welding quality cannot be set to a severe value, resulting in that criteria for checking the welding quality may be lenient (even “bad” or “poor” welting quality may be judged as “good”).